R&D: Nanoparticle Films for High-Density Storage

New holographic data storage medium could enable wearable technology that captures and stores detailed 3-D images

Researchers created nanofilm that can store data
holographically and is environmentally stable.
Here, Shencheng Fu carries out experiments with new film.
(Credit: Northeast Normal University)

As we generate more and more data, the need for high-density data storage that remains stable over time is becoming critical. New nanoparticle-based films that are more than 80 times thinner than a human hair may help to fill this need by providing materials that can holographically archive more than 1000 times more data than a DVD in a 10-by-10-centimeter piece of film. The new technology could one day enable tiny wearable devices that capture and store 3-D images of objects or people.

“In the future, these new films could be incorporated into a tiny storage chip that records 3-D color information that could later be viewed as a 3-D hologram with realistic detail,” said Shencheng Fu, who led researchers, Northeast Normal University, China who developed the new films. “Because the storage medium is environmentally stable, the device could be used outside or even brought into the harsh radiation conditions of outer space.”

In the journal Optical Materials Express, the researchers detail their fabrication of the new films and demonstrate the technology’s ability to be used for an environmentally-stable holographic storage system. The films not only hold large amounts of data, but that data can also be retrieved at speeds up to 1GB /s, which is about twenty times the reading speed of today’s flash memory.

Storing more data in less space
The new films are designed for holographic data storage, a technique that uses lasers to create and read a 3-D holographic recreation of data in a material. Because it can record and read millions of bits at once, holographic data storage is much faster than optical and magnetic approaches typically used for data storage today, which record and read individual bits one at a time. Holographic approaches are also inherently high-density because they record information throughout the 3-D volume of the material, not just on the surface, and can record multiple images in the same area using light at different angles or consisting of different colors.

Recently, researchers have been experimenting with using metal-semiconductor nanocomposites as a medium for storing nanoscale holograms with high spatial resolution. Porous films made of the semiconductor titania and silver nanoparticles are promising for this application because they change color when exposed to various wavelengths, or colors, of laser light and because a set of 3-D images can be recorded at the focus area of laser beam using a single step. Although the films could be used for multiwavelength holographic data storage, exposure to UV light has been shown to erase the data, making the films unstable for long-term information storage.

Recording a holographic image into titania-silver films involves using a laser to convert the silver particles into silver cations, which have a positive charge due to extra electrons. “We noticed that UV light could erase the data because it caused electrons to transfer from the semiconductor film to the metal nanoparticles, inducing the same photo transformation as the laser,” said Fu. “Introducing electron-accepting molecules into the system causes some of the electrons to flow from the semiconductor to these molecules, weakening the ability of UV light to erase the data and creating an environmentally stable high-density data storage medium.”

Changing the electron flow
For the new films, the researchers used electron-accepting molecules that measured only one to two nanometers to disrupt the electron flow from the semiconductor to the metal nanoparticles. They fabricated semiconductor films with a honeycomb nanopore structure that allowed the nanoparticles, electron-accepting molecules and the semiconductor to all interface with each other. The ultrasmall size of the electron-accepting molecules allowed them to attach inside the pores without affecting the pore structure. The final films were just 620 nanometers thick.

The researchers tested their new films and found that holograms can be written into them efficiently and with high stability even in the presence of UV light. The researchers also demonstrated that using the electron-acceptors to change the electron flow formed multiple electron transferring paths, making the material respond faster to the laser light and greatly accelerating the speed of data writing.

“Particles made from noble metals such as silver are typically viewed as a slow-response media for optical storage,” said Fu. “We show that using a new electron transport flow improves the optical response speed of the particles while still maintaining the particle’s other advantages for information storage.”

The researchers plan to test the environmental stability of the new films by performing outdoor tests. They also point out that real-life application of the films would require the development of high efficiency 3-D image reconstruction techniques and methods for color presentation for displaying or reading the stored data.

Article : UV-resistant holographic data storage in noble-metal/semiconductor nanocomposite films with electron-acceptors

Optical Materials Express has published an article written by Shuangyan Liu, Shencheng Fu, Xintong Zhang, Xinnong Wang, Lihong Kang, Xiuxiu Han, Xi Chen, Jiarui Wu, and Yichun Liu, Center for Advanced Optoelectronic Functional Material Research, Northeast Normal University and Key Laboratory of UV-Emitting Materials and Technology (Northeast Normal University), Ministry of Education, Changchun 130024, China.

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Abstract: “Metal/semiconductor nanocomposite systems with the ability of controllable holographic storage are fascinating for advancing information technology. Ag/TiO₂ nanocomposite films present multicolor photochromism, which plays a key role in high-density optical memory. However, the film undergoes a reversible photo-redox reaction by the alternating action of visible and UV lights, which weakens the optical stability of stored information. To date, no effective method has been proposed to hinder the UV-erasure in the film. In this paper, the transferring behavior of electrons in a Schottky junction between Ag and TiO₂ is inhibited effectively by introducing electron acceptors into the photochromic film. Plasmonic photo-dissolution is enhanced greatly, which is in accordance with the theoretical fitting based on the reversible photo-chemical reaction. Holograms can be written efficiently with high stability even under the destructive UV-irradiation, which are expected to be applied in an environmentally-stable photo-device“ .